1. Field of the Invention
This invention relates to a method for mounting a microsphere coupling lens on an optical fiber, especially for providing the efficient coupling of light to the fiber. Accordingly, it is a general object of this invention to provide new and improved methods of such character.
2. Description of the Prior Art
The combination of spherical lenses with optical fibers, or, specifically, the use of microsphere bead lenses for diode laser-to-laser coupling, has been discussed in the prior art in various printed publications. Some of these printed publications include:
G. D. Khoe and L. J. Meuleman, "Light modulation and injection in optical-fibre transmission systems with semiconductor lasers", Philips Tech. Rev. 36 (1976), pp. 201-204, discusses the coupling of glass fibers to lasers. A spherical lens is inserted in front of the entrance plane of a glass fiber. The lens converges laser light to an almost parallel beam, mainly incident within the acceptance angle of the fiber. The core of the fiber and its cladding are housed within a glass capillary tube. The lens is cemented to the capillary tube.
G. D. Khoe and G. Kuyt, "A Luneberg lens for the efficient coupling of a laser diode and a graded-index fibre", Proc. of the 3rd European Conference on Optical Communication, Munich, September 1977, pp. 176-178, states that glass beads can be glued on the fibers using a simple tool.
G. K. Khoe and G. Kuyt, "Realistic efficiency of coupling light from GaAs laser diodes into parabolic-index optical fibres", Electronic Letters, Vol. 14 (1978) pp. 667-669, states that microscopic glass beads are glued on the fiber end of graded-index fibers using U.V.-cured epoxy. A stated advantage of microbeads (over cylindrical lenses) is that orientation is arbitrary.
G. D. Khoe, H. G. Kock, and L. J. Meuleman, "Fiberless hermetic packaged lens-coupled laser diode for wideband optical-fiber transmission", Proc. Topical Mtg. On Opt. Fiber Communication, Washington, D.C., March 1979, pp. 94-97, discusses an optical configuration derived from the foregoing scheme in which a glass bead with a high refractive index is glued on the inside glass window of a modified TO-15 package. The position of the window with the lens on it is then adjusted to a laser. A bright spot of the laser is displayed in combination with a circular pattern originating from the microlens. Proper transverse alignment is obtained by adjusting the bright spot to the center of the pattern. Then, the cap is welded to the base.
Microsphere bead lenses are desirable in that they can produce as high as 65% to 80% coupling efficiencies (which are among the highest of various coupling techniques that are presently available). Such lenses provide for laser-lens alignment tolerances that are workable in production.
For a multimode fiber, a microsphere bead coupling lens has a typical diameter in the range of 60 microns to 125 microns and is usually made of glass (having an index n.sub.c up to 1.91). The bead is attached to the end of an optical fiber with a transparent adhesive such as a transparent epoxy. The bead acts as a lens which directs widely diverging rays from the laser into the fiber core at an angle that is within the fiber's acceptance angle. Thus, it can be an efficient coupling lens.
However, to achieve a reproducibly high coupling efficiency with such a lens, the bead must be very well centered on the end of the fiber, usually to within 5 microns. Preferably, in its mounting, the epoxy must not run over onto the front or laser-facing bead surface. The lens, preferably, should end up close to the fiber, i.e, within a few microns.